1. Field of the Invention
This application claims priority to Korean Patent Application No. 10-2009-0014357, filed on Feb. 20, 2009, the entirety of which is incorporated herein by reference.
The present invention relates to a polymer composition for an insulation material of electric cables. In particular, the present invention relates to an ethylene copolymer-based composition for an insulation material.
2. Description of the Related Art
When manufacturing an insulation of electric cables bent to be installed in narrow spaces, for example electric cables for ships, it is important to consider all of electrical properties such as insulation resistivity and voltage resistant characteristics, mechanical properties such as high tensile strength, and heat resistance for protection from fire, which are required for an insulation.
Conventionally, a crosslinked polyethylene (XLPE)-based insulation having crystallinity has been used to ensure said electrical, mechanical and chemical characteristics required for electric cables installed in narrow spaces such as ships. Crystalline polyethylene is mainly used to ensure a required level of tensile strength. Typically, crystalline XLPE may be, for example, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) or middle low density polyethylene (MDPE). These are crystalline resins of which a melting point is generally 100° C. or above. Conventional insulated electric cables comprise an insulation around a conductor, manufactured using said crystalline polymer-based composition, and a sheath formed by extruding a sheath material and crosslinking in an in-line manner under high temperature and high pressure conditions (e.g., pressure of 8 bar and temperature of 180° C.). However, during crosslinking of sheath material under high temperature and high pressure conditions, the insulation may melt due to heat of a vulcanization tube and be deformed and pressed into lumps due to ambient high pressure steam. When the insulation is pressed into lumps, the insulation cannot maintain a circular shape basically required, resulting in a quality problem associated with the basic appearance. And, it is impossible to ensure the electrical properties required very importantly for the insulation, and thus, if a high-tension electricity is applied, the insulation is broken.
To solve the problems, conventional techniques have been suggested in which insulated electric cables are produced with a crystalline resin by crosslinking a sheath in a batch manner, not in an in-line manner, after extrusion of the sheath. The crosslinking in the batch manner is performed at lower temperature than a melting point where thermal deformation does not occur. However, the techniques disadvantageously require a very long time, for example 3 hours or longer to produce radicals required for crosslinking because a crosslinking agent is dissolved at a low temperature.
Crystalline polyethylene has strong attractive forces between molecules, and thus is advantageous in terms of tensile strength, but has high flexural strength at normal temperature. For this reason, electric cables having an insulation made from crystalline polyethylene are not flexible. In electric cables (or cables) for control and signaling, most of a plurality of electric cables are complicatedly installed in cabinet panels, distributing boards or control boxes. It is very difficult to strip coatings from and branch off said electric cables while connected in narrow spaces so as to change connection. It is more difficult to do so because a majority of electric cables (or cables) for power, control, signaling and so forth have an inner metal line, i.e., a braided shield to prevent deformation caused by tension and lateral pressure applied thereto during installation. Thus, conventional electric cables are difficult to install due to low flexibility and need much energy to bend in narrow spaces.
Meanwhile, a crystalline resin is resistant against external shocks, but deforms when shocks exceeding a predetermined level are applied. Higher crystallinity results in higher density. In this case, shock resistance and flexibility at low temperature reduces, and elasticity is low. Thus, due to low elasticity, once a crystalline resin deforms, it cannot restore to the original shape but remains deformed. When applied to cables, a crystalline resin is generally extruded over a conductor to form a circular shape in cross section. If the circular shape of the insulation deforms due to external shocks, the insulation cannot retain the insulation characteristics of XLPE required for cables, causing electrical problems such as dielectric breakdown and so forth. The dielectric breakdown involves high temperature, and consequently, a fire breaks out at the insulation, possibly resulting in conflagration.
In conclusion, insulated electric cables having a crystalline polyethylene-based insulation suffer sacrifice of productivity during a crosslinking process, and need much efforts, techniques and costs to install. And, they have low elasticity, and thus are difficult to restore to the original shape, which acts as one of safety-threatening factors.